Physical Layer Error Handling

When the Physical Layer logic detects an error, it sends a Receiver Error indication to the Data Link Layer. The specification lists a few of these errors, but it is far from being an exhaustive error list. It is up to the designer to determine what Physical Layer errors to detect and report.

Some of these errors include:

• 8b/10b Decoder-related disparity errors (described in "Disparity Errors" on page 447). This check is required.

• 8b/10b Decoder-related code violation errors (described in "Code Violations" on page 446). This check is required.

• Elastic Buffer overflow or underflow caused by loss of symbol(s) (described in "The Elastic Buffer's Role in the Receiver" on page 442).

• The packet received is not consistent with the packet format rules described in "General Packet Format Rules" on page 411. This condition is optionally checked.

• Loss of Symbol Lock (see "Symbol Boundary Sensing (Symbol Lock)" on page 441).

• Loss of Lane-to-Lane de-skew (see "Lane-to-Lane De-Skew" on page 444).

Response of Data Link Layer to 'Receiver Error' Indication

If the Physical Layer indicates a Receiver Error to the Data Link Layer, the Data Link Layer discards the TLP currently being received and frees any storage allocated for the TLP. The Data Link Layer schedules a NAK DLLP for transmission back to the transmitter of the TLP. Doing so automatically causes the transmitter device to replay TLPs from the Replay Buffer, resulting in possible auto-correction of the error. The Data Link Layer may also direct the Physical Layer to initiate Link re-training (i.e., link recovery).

Detected Link errors may also result in the Physical Layer initiating the Link retraining (recovery) process.

In addition, the device that detects a Receiver Error sets the Receiver Error Status bit in the Correctable Error Status register (see Figure 24-20 on page 936) of the PCI Express Extended Advanced Error Capabilities register set. If enabled to do so, the device sends an ERR_COR (correctable error) message to the Root Complex (See "Advanced Uncorrectable Error Handling" on page 386 for details on error logging and reporting).